Zinc
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Characteristics Physical Zinc, also referred to in nonscientific contexts as spelter, encyclo.co.uk is a bluish-white, lustrous, diamagnetic metal, 2006 though most common commercial grades of the metal have a dull finish. 1992| p.123 It is somewhat less dense than iron and has a hexagonal crystal structure. 1968| p.826 The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C.2006 1992| p.123 Above 210 °C, the metal becomes brittle again and can be pulverized by beating. 1861 Zinc is a fair conductor of electricity. 2006 For a metal, zinc has relatively low melting (420 °C) and boiling points (900 °C). galvanizeit.org Its melting point is the lowest of all the transition metals aside from mercury and cadmium. galvanizeit.org Many alloys contain zinc, including brass, an alloy of zinc and copper. Other metals long known to form binary alloys with zinc are aluminium, antimony, bismuth, gold, iron, lead, mercury, silver, tin, magnesium, cobalt, nickel, tellurium and sodium. 1902 While neither zinc nor zirconium are ferromagnetic, their alloy ZrZn2 exhibits ferromagnetism below 35 K. 2006 Occurrence Zinc makes up about 75 ppm (0.007%) of the Earth's crust, making it the 24th most abundant element there. 2001|p.503 Soil contains 5–770 ppm of zinc with an average of 64 ppm.2001|p.503 Seawater has only 30 ppb zinc and the atmosphere contains 0.1–4 µg/m3. 2001|p.503 (ZnS)|alt=A black shiny lump of solid with uneven surface.]] The element is normally found in association with other base metals such as copper and lead in ores.1968| p.822 Zinc is a chalcophile ("sulfur loving"), meaning the element has a low affinity for oxygen and prefers to bond with sulfur in highly insoluble sulfides. Chalcophiles formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. 1997| p.1202 Sphalerite, which is a form of zinc sulfide, is the most heavily mined zinc-containing ore because its concentrate contains 60–62% zinc. 1968| p.822 Other minerals, from which zinc is extracted, include smithsonite (zinc carbonate), hemimorphite (zinc silicate), wurtzite (another zinc sulfide), and sometimes hydrozincite (basic zinc carbonate).2001|p.502 With the exception of wurtzite, all these other minerals were formed as a result of weathering processes on the primordial zinc sulfides. 1997| p.1202 World zinc resources total about 1.8 gigatonnes.2009 Nearly 200 megatonnes were economically viable in 2008; adding marginally economic and subeconomic reserves to that number, a total reserve base of 500 megatonnes has been identified. 2009 Large deposits are in Australia, Canada and the United States.1997| p.1202 At the current rate of consumption, these reserves are estimated to be depleted sometime between 2027 and 2055.2007 idtechex.com About 346 megatonnes have been extracted throughout history to 2002, and one estimate found that about 109 megatonnes of that remains in use. 2006 Biological role Zinc is an essential trace element, necessary for plants,2007 animals, A. S., 2008 and microorganisms.1983 Zinc is found in nearly 100 specific enzymes 2000| p.443 (other sources say 300), serves as structural ions in transcription factors and is stored and transferred in metallothioneins.1999| pp.625–629 It is "typically the second most abundant transition metal in organisms" after iron and it is the only metal which appears in all enzyme classes.2007 There are 2–4 grams of zinc2000 distributed throughout the human body. Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye.1990| p.131 Semen is particularly rich in zinc, which is a key factor in prostate gland function and reproductive organ growth.2007| p.210 In humans, zinc plays "ubiquitous biological roles".2007 It interacts with "a wide range of organic ligands",2007 and has roles in the metabolism of RNA and DNA, signal transduction, and gene expression. It also regulates apoptosis. A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds which transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins.2007 In the brain, zinc is stored in specific synaptic vesicles by glutamatergic neurons 2009 and can "modulate brain excitability".2007 It plays a key role in synaptic plasticity and so in learning.2009 However it has been called "the brain's dark horse"2009 since it also can be a neurotoxin, suggesting zinc homeostasis plays a critical role in normal functioning of the brain and central nervous system.2009 Bibliography * 2009 * 2009 * 2007 * 2003 * *encyclo.co.uk * 2006 * 1992 * 1968 * 1861 * *galvanizeit.org * 1902 * 2001 * 1997 * 2009 * 2007 * *idtechex.com * 2006 * 2007 * A. S., 2008 * 1983 Zinc's role in microorganisms is particularly reviewed in: * 2000 * 1999 * 2000 * 1990 * 2007 en Категория:SciRefs